refactor: delete two orphans, and record why the reparent design changed

`resolveVideoSource` chose between a local file and a remote URL for video
playback. Backend-owned stream selection took that decision into Rust —
`media_local_selection` for a downloaded file, `get_stream_selection` for a
streamed one — and its last caller went with it. What remained was the function
plus sixty lines of tests exercising nothing that ships.

`fittedVideoSize` computed the rendered size of a video letterboxed into its
container. Nothing has ever called it: it arrived with the fix that made the
video fill its viewport and was superseded by `object-fit: contain` in the same
change. There is some irony in a helper that models letterboxing sitting unused
beside a container that was not letterboxing at all — the bug fixed in the
previous commit was CSS, and this function would not have helped.

A survey for exported symbols referenced only by their own tests finds 22 more.
Most are legitimate — test mocks, deliberate reset hooks, public utility APIs —
and the rest are unrelated to this work, so they are left for a cleanup that can
be reviewed on its own terms rather than smuggled into a playback branch.

Also records in the spec why DR-231's design changed. Reparenting Tauri's
webview into a GtkOverlay aborts the process on the first click: Linux calls
`attach_resize_handler` unconditionally (the Windows path guards it with
`is_decorated()`), and its handler walks webview -> GtkBox -> GtkWindow with an
unwrap that an overlay breaks. So the webview is not moved at all — mpv draws
into the default vbox's own `draw` handler via `gdk_cairo_draw_from_gl()`, and
GTK's container-before-children order puts the webview on top for free. No
reparent, one less widget, and nothing a Tauri upgrade can invalidate by
assuming its own layout.

src/lib/player/localSource.ts

@@ -1,41 +1,3 @@
-/**
- * Choosing between a downloaded file and a server stream for video playback.
- *
- * Audio has preferred local files since the queue is built (the Rust queue
- * resolves `MediaSource::Local`), but video asks the repository for a stream URL
- * and never consults `downloads` — so a downloaded film was streamed anyway,
- * spending bandwidth that had already been spent and failing outright offline.
- *
- * Pure so it can be unit-tested: the component only supplies the two inputs and
- * the asset-URL converter.
- *
- * TRACES: UR-071 | DR-123 | UT-118
- */
-
-export interface VideoSourceInputs {
-  /** Absolute on-disk path of a completed download, or null to stream. */
-  localPath: string | null;
-  /** Stream URL the repository resolved (already transcoded if it had to be). */
-  remoteUrl: string;
-  /** Whether the *remote* stream is a transcode. */
-  remoteNeedsTranscoding: boolean;
-  /** Usually Tauri's `convertFileSrc`; injected so this module stays pure. */
-  toAssetUrl: (path: string) => string;
-}
-
-export interface VideoSourceDecision {
-  /** What to hand the `<video>` element. */
-  url: string;
-  /**
-   * Local files are never transcodes, so this is always false for them. It
-   * matters because the transcoded path re-requests a whole new stream URL on
-   * every seek; a local file seeks natively and must not go down that route.
-   */
-  needsTranscoding: boolean;
-  /** True when playing from disk — for logging and the offline badge. */
-  isLocal: boolean;
-}
-
 /** Absolute on POSIX (`/…`), Windows (`C:\…`, `C:/…`) or a UNC share (`\\…`). */
 function isAbsolute(path: string): boolean {
   return path.startsWith("/") || path.startsWith("\\") || /^[A-Za-z]:[\\/]/.test(path);
@@ -57,15 +19,3 @@ function isAbsolute(path: string): boolean {
 export function downloadedFilePath(storageRoot: string, filePath: string): string {
   return isAbsolute(filePath) ? filePath : `${storageRoot}/${filePath}`;
 }
-
-export function resolveVideoSource(inputs: VideoSourceInputs): VideoSourceDecision {
-  const { localPath, remoteUrl, remoteNeedsTranscoding, toAssetUrl } = inputs;
-
-  // Treat blank/whitespace paths as absent — a malformed `downloads` row must
-  // not produce an asset URL pointing at nothing.
-  if (localPath && localPath.trim() !== "") {
-    return { url: toAssetUrl(localPath), needsTranscoding: false, isLocal: true };
-  }
-
-  return { url: remoteUrl, needsTranscoding: remoteNeedsTranscoding, isLocal: false };
-}